In recent years, there has been increasing trends in higher processing speed of digital circuits, and increasing needs for high-density packaging. Signal lines on printed circuit boards used for these circuits are matched in impedance so as to adjust the characteristic impedance Z0 to a constant value, in order to prevent transmission loss and degradation of communication quality. The characteristic impedance Z0 is defined by a square root of (L/C) as given by the equation (1) below:(Mathematical Formula 1)Z0=√(L/C)  (1)
where, Z0 is characteristic impedance, L is inductance, and C is capacitance.
The characteristic impedance Z0 is determined by physical shapes such as the width of signal line, the thickness of signal line, or the distance between the signal line and a ground layer, or physical characteristic values such as the dielectric constant of an insulating resin layer composing the printed circuit board, and the conductivity of the conductor layer.
For example, as for matching of the characteristic impedance Z0 of single-ended line, the characteristic impedance is generally controlled to 50Ω.
By the way, in recent years, there has been a need for increase in signal speed of the printed circuit board. To cope with the need, physical shape of an arbitrary configuration in the printed circuit board is controlled. This successfully satisfies the need to some degree. Typically, the characteristic impedance Z0 can be matched by increasing the width of the signal line and by increasing the thickness of the insulating resin layer, so as to reduce the capacitance.
On the contrary, there has also been a need for down-sizing of devices. Therefore, despite the need for matching the characteristic impedance Z0 so as to reduce the transmission loss under high-speed signal transmission, the insulating resin layer has been increased in the thickness only to a limited degree.
For example, smartphone capable of handling high-speed signals equivalent to those in personal computer has widely disseminated. This sort of smartphone suffers from a trade-off between portability and provision of a sufficient battery capacity. The smartphone is therefore required to reduce the occupancy by components other than the battery in the instrument. Accordingly, the thickness of flexible printed circuit board with transmission lines has been becoming more stringently restricted than before.
Patent Literature 1 discloses a porous polyimide film with copper foil, composed of a copper foil and a porous polyimide film (see [FIG. 1] of this literature). The porous polyimide film contains micro-bubbles with a void diameter of approximately 0.2 μm, and has a porosity adjusted to 50% or around (see paragraphs [0060] to [0063] of this literature). Patent Literature 1 describes that the polyimide film may be reduced in the dielectric constant, by introducing isolated pores to make the film porous.
Patent Literature 2 discloses a strip transmission line substrate which includes a copper foil having thereon a strip conductor, and an aluminum ground substrate opposed to the copper foil while placing an air layer in between (see [FIG. 10] of this literature). The strip conductor is composed of an electrolytic copper plated layer and an electroless nickel plated layer, and has on the surface thereof an electroless gold plated layer. According to the description in paragraph [0024] of this literature, the air layer has a small dielectric loss, and is therefore effective as a dielectric layer opposed to the strip conductor. This paragraph of this literature also describes that the ground substrate provided while placing the air layer in between contributes to reduce radiation loss, and synergistically with an effect of using the air layer as a dielectric layer, also contributes to improve the transmission characteristic.